The aerodynamics of ground vehicles are adversely influenced by a number of factors, such as separated air flow-fields and vortex formation and shedding. When a ground vehicle encounters a relative wind, such as when the vehicle is in motion, the air travels along the surface of the vehicle and typically separates from the vehicle at the rear portion of the vehicle. The separated flow-fields cause an increased drag on the vehicle, increased fuel consumption, and an increased level of noise perceived within the interior of the vehicle, plus a loss in cruise efficiency and in economic operation. Resulting lift can positively or adversely effect the vehicle's performance and control.
In general, the vortex formation and shedding is the result of a flow around curved surfaces between differential upper and lower surface pressure distributions. With ground vehicles, a vortex formation is caused by unlike pressure distributions between the upper and lower surfaces of the ground vehicle which induces a drag on the vehicle. The drag increase caused by the vortex thus formed varies in proportion to the square of the lift coefficient experienced by the vehicle and in proportion to the square of the vehicle's velocity.
Because of the above-noted problems, ground vehicles have been designed to be more streamlined so as to reduce the amount of flow separation and vortex formation. While recent ground vehicles are much more streamlined than past models, the recent designs still have a value of practical vehicle drag which is much higher than that of an ideal streamlined body. Thus, today's ground vehicles still experience a significant amount of flow separation and vortex formation.
Some additional aerodynamic problems are related to the sensitivity of a ground vehicle to side winds or to the vehicle being yawed relative to the flow direction, such as when the vehicle is cornering or when it receives gusts from passing vehicles. These situations, such as when the car is cornering or receiving side gusts, can yield both directional instabilities about a vertical yaw axis and lateral instabilities about a horizontal roll axis. As a result of these directional instabilities about the yaw and roll axes, a driver of the vehicle may have difficultly in maintaining control at any time that the relative wind is not from straight ahead of the vehicle. There is therefore a need to provide greater stability to a ground vehicle when it receives relative wind from a direction other than from straight ahead of the vehicle.
The variation in yaw-sensitivity for a standard car and for a streamlined car is plotted in FIG. 1(A). As evident from FIG. 1(A), the standard sharp-cornered car is stable even up to 35.degree. of yaw angle because the flow of air around the front corners of the car separates and prevents increasing side loads and yawing moment. The streamlined car, in contrast, begins to show rapid aerodynamic force changes at yaw angles as low as 20.degree. and therefore experiences lateral or directional instabilities. Since recent designs of cars are more streamlined, the cars tend to become unstable at lower angles of yaw.
A further problem experienced by ground vehicles is ground effect which is caused by the proximity of the lower surface of the car to the roadway over which it is moving. The problem of ground effect can dramatically change the flow-field around the lower surface of the car compared to its "free-stream" flow pattern. As evident from FIG. 1(B), depending upon the particular design of the ground vehicle, both the lift and drag can increase or decrease significantly as the clearance between the ground and the lower surface of the vehicle is reduced. The ground effect, however, is rather unpredictable and is difficult to control.
Lift generated by a streamlined car can also cause control problems; it can effectively reduce vehicle weight and thus reduce tire traction and cornering ability. Some high speed vehicles employ spoilers or inverted airfoils to reduce this lift. Both will cause increased download to increase tire traction and handling. However, the increased lift and reducing rolling resistance can improve cruise efficiency.
The various aerodynamic forces acting upon a ground vehicle, such as flow separation, vortex formation, side winds, and ground effect, can dramatically influence the performance, economy of operation and control of the ground vehicle. Some conventional approaches to the problem have included designing the vehicles with more streamlined shapes, equipping them with larger wheels to remove the lower surface of the car from the road to decrease ground effect, and the use of numerous mechanical devices, such as air dams and spoilers.
While these conventional approaches may reduce to some effect the above-noted problems, each of the approaches is limited in its effectiveness or desirability. For instance, streamlined shapes may be too limiting in the internal dimensions of the vehicle, the passenger and payload room, as well as resulting in a more expensive vehicle to manufacture. While larger wheels may reduce the ground effect in some vehicles, the larger wheels may cause lateral instability due to the high center of gravity thus created and may increase the risk of roll-over for the vehicle. As for mechanical aerodynamic controls, these devices add weight and undue complexity to the vehicle. Further, since the mechanical controls usually cause drag at off-design conditions, e.g., lower or higher speeds, the mechanical controls desirably must be mechanically adjustable, which further adds to the weight and complexity.
Another approach in improving the aerodynamics of a ground vehicle involves redirecting air through portions of the vehicle. U.S. Pat. No. 3,437,371 to Gallie et al. provides an example of such a system in which air is drawn in through slots at the rear portion of the vehicle and is routed through pipes to the front of the vehicle. The front area of the vehicle is at a low pressure and provides the suction force to draw the air in through the slots in the rear of the vehicle thereby to reduce the amount of flow separation. This system has minimal benefit on the aerodynamics of the vehicle inasmuch as the amount of suction force generated at the front of the vehicle will likely at most speeds be insufficient to alter the flow at the rear of the vehicle, especially in view of the amount of friction generated within the air pipes or ducts. This system further suffers from a disadvantage that the suction forces cannot be controlled independently of the vehicle's speed.
Another type of system reduces the flow separation by blowing air out of a region of the vehicle thus eliminating reliance on suction or low pressure areas. For instance, U.S. Pat. No. 5,407,245 to Geropp discloses a system in which air is drawn in through openings in a rear region of the vehicle and is blown out along lines of separation of the vehicle. The system uses a pair of blowers to draw air in through openings formed in the rear surface of the vehicle and to force the air out of bores or slots at or near the separation region. This type of system still depends upon recirculating air through the car but is better able to reduce flow separation and the amount of drag on the vehicle.
The system disclosed in the Geropp patent, while reducing the flow separation, has several apparent disadvantages. The system employs blowers which add a further drain on the vehicle's power system and present another mechanical component requiring maintenance or repair. Also, the system is not easily incorporated into existing cars as it is located at the very rear of the vehicle and necessarily reduces the amount of available trunk space. Furthermore, a rear bumper and its associated structure, which are apparently not taken into consideration by Geropp, can significantly impair the amount of air drawn in through the openings and the ability of the system to reduce flow separation. Thus, while the Geropp system would be able to reduce flow separation, the system is apparently not optimally designed for existing automobiles.
The conventional systems designed to improve the aerodynamics of a vehicle have been primarily limited to reducing drag by reducing the amount of flow separation. Some of the other aerodynamic problems in a vehicle, such as yaw sensitivity, vortex formation, and ground effect, have been mainly ignored.